This invention relates to workholding chucks as used on automatic turning machines. Automatic turning machines generally utilize long bars for work feed stock which extend through the spindle of the head stock of the machine out of the front of the spindle and though a work holding chuck. As a workpiece is finish-machined and cut off from the parent bar stock, the grip of the workholding chuck can be relaxed and the bar is fed outward to present new material for machining. Many types of bar feeder attachments are used to increment stock through the work spindle. There are pushers which urge the workstock forward from the rear of the headstock and there are pull type attachments which grab a part and withdraw it from the front of the chuck. It is with this latter type of attachment that the present invention can be used most advantageously.
After withdrawing additional stock through the chuck for the next successive machine operation the chuck reclamps on the workstock. At this time, it is imperative that there be sufficient clamping stock present within the clamping area. If this condition is not met and there is not sufficient clamping stock present in the clamping area, a real and active danger exists, in that as the workpiece is rotated and a cutting tool is engaged against the workpiece, it may be "cammed" out of the clamping position due to forces generated by the cutting action. Additionally, in the absence of sufficient clamping stock, the chuck may not be operating at its proper design forces and may tend to have overstressed points which could operate to the detriment of the chuck assembly and cause failure of the components. For example, if a jaw type chuck is used for clamping the work stock and there is not sufficient stock to cover the clamping face of the jaws, high unit stresses will be developed over the zone of the clamp and high bending stresses may be developed in the jaw bodies as well. Since the jaws are normally subjected to great stresses because of the fly-away tendencies of centrifugal action, the internal stresses are compounded and may cause failure of the jaws.
Most prior art devices for sensing the presence of stock to allow adequate clamping have also included cumbersome mechanical devices which reach through the headstock spindle and touch the end of the workpiece. These devices, however, are impractical to use and function. Similarly, stock feelers have been contained within a chuck body which necessitates having a complex chuck unit and feedback sensing control. Many of these disadvantages have been alleviated by the stock sensing device disclosed in applicant's U.S. Pat. No. 3,982,085. The present invention, likewise, alleviates many of these prior art problems with further elimination of mechanical features and increased simplicity. It provides an alternative method of stock sensing with additional cost savings.